The present disclosure relates generally to the field of an aircraft weather radar system for assessing weather and ground clutter. The disclosure more specifically relates to an aircraft weather radar system capable of compensating for atmospheric refraction.
Aircraft weather radar systems are used to assess weather altitudes and to separate weather from ground clutter. In conventional aircraft weather radar systems, changes in atmospheric refraction produce radar beam bending, which in turn causes variations in weather and ground height estimations. These variations cause errors in the assessment of weather altitudes as well as in the weather and/or clutter separation process.
Currently, the MultiScan® radar system manufactured by Rockwell Collins, Inc. uses a dual beam system to generate change in power estimates from targets within its field of view. For a given target the change in power estimates between the two beams is determined by the target's angular position in the beam set. The farther a target is below the center of the radar beam set, the larger the difference in return power between the two beams. For ground targets, the targets' perceived angular position in the beam set is a function of the aircraft altitude, physical beam pointing angles, geometry to the curved Earth's surface, and bending of the radar's beam along the path to and from the Earth. This bending, caused by changes in atmospheric density, is known as refraction. For ground based radars there is an accepted approximation for estimating the bending of a radar's beam through the atmosphere due to “standard atmospheric conditions”. This approximation assumes the atmosphere's density and water content is constant but the Earth's diameter is 4/3 times the Earth's actual diameter. While generally useful, this approximation is not effective in all conditions and locations.
In extreme cases, a high vertical gradient in moisture content may cause a radar beam pointed above the horizon to bend downwards and hit the ground at some long range. This produces spurious ground returns from anomalous propagation or “ducting”.
Various and somewhat unpredictable atmospheric conditions can make compensation for beam bending difficult. For example, the atmosphere's moisture gradient can contribute to the amount of beam bending (e.g., moisture from rain may result in increased radar beam refraction).
Therefore, there is a need for an aircraft weather radar system that can more accurately predict weather at medium to long ranges. There is also a need for weather and ground return processing that more accurately separates weather from ground clutter. There is a further need for a weather system that may detect the altitude of weather and ground clutter with increased accuracy. Further still, there is a need for a system and method of compensating for the affects of refraction on the radar beam. Yet further, there is a need for a system and method that compensates for “beam bending” or ducting.